The present invention relates generally to improved apparatus for a center pivot irrigation system and more particularly to improved apparatus for equalizing the forces resulting on truss members supporting the system pipeline between mobile tower assemblies transporting the pipeline across uneven terrain.
Center pivot irrigation systems and equipment, such as that described in my copending United States patent application, Ser. No. 778,069, filed Mar. 16, 1977, generally comprise a plurality of water sprinklers mounted along the length of a series of coupled pipe sections which, in turn, are transported on a plurality of mobile tower assemblies. Water is pumped through the connected pipe sections, and water is sprayed onto the field through the sprinklers as the mobile tower assemblies with the pipes supported thereon rotate about the center pivot point. The pipeline, in many instances, extends up to one-half mile in length, and accordingly, when the pipeline is rotated about the center pivot point on the mobile tower assemblies, the system is capable of irrigating fields approximately one mile or less in diameter.
In order to minimize crop damage by the supportive tower assemblies, the assemblies are spaced at wide intervals along the pipeline. Because of the large quantities of water required for irrigation and the resulting weight associated therewith, however, additional apparatus is usually provided to support the pipeline between adjacent tower assemblies. Such apparatus commonly includes a series of triangularly-shaped trusses attached to the pipeline, for example, by bolting to a mounting bracket welded to the pipeline, and supported beneath the pipeline by underslung support members such as rods or cables extending between adjacent tower assemblies. As water is transported through the pipeline, the pipeline bears downwardly on the trusses which, in combination with the support means, support the pipeline between the tower assemblies. Such supportive apparatus increases the load bearing capabilities of the pipeline, permits the use of large diameter pipeline, and in addition, the tower assemblies can be spaced farther apart than would otherwise be possible.
One problem associated with the above described arrangement, however, is that the forces exerted on the truss members by the pipeline become unbalanced when the pipeline is transported over uneven terrain. In particular, when the tower assemblies are moving across substantially level ground, the support rods and the trusses are vertically symmetrical with respect to the pipeline, and the forces resulting on the two slanting side members of each particularly triangularly-shaped truss are substantially equal. However, as the tower assemblies follow the contours of the land and climb or descend sloped terrain, as the case may be, the support rods and the trusses tend to be correspondingly displaced or swung from their normal vertically symmetrical position with respect to the pipeline. This causes an imbalance between the forces bearing on the side members of the truss with a greater force being brought to bear on one side member than on the other side member. In certain cases, this may result in excessive stress being put on one or more of the truss members sufficient to damage the system.
Heretofore, in order to equalize the forces on the truss side members, a rigid horizontal cross member connecting the side members at an intermediate point along their lengths has been included. Typically, the intermediate cross member is located parallel to and above the bottom horizontal truss member and spaced downwardly from the pipeline. The intermediate horizontal member serves to establish fulcrums at the intermediate contact points on the side members. Thus, when the pipeline is being transported uphill, for example, a strong force in the direction opposite to the direction of travel of the apparatus is applied to the bottom end of the forwardly disposed side member by the rearwardly disposed side member through the horizontal bottom member of the triangularly-shaped truss. The fulcrum established at the forward member by the intermediate horizontal truss member causes the resultant force applied to the pipeline at the other end of the forward side member to be in a forward direction thereby tending to return the support apparatus to its normal vertically symmetrical position with respect to the pipeline and equalize the forces applied to the side members.
Conversely, if the apparatus is moving downhill, the force applied to the pipeline by the rearwardly-disposed side member acting through the fulcrum is in the opposite or clockwise direction, i.e., toward the rear, causing the support apparatus to tend to return to its generally substantially upright position and thus neutralizing the force imbalance on the side truss members.
Installation of the intermediate horizontal cross member on each one of the many trusses required for a single center pivot irrigation system, however, greatly increases the material costs of the system as well as the labor costs associated with fastening the members to the trusses and welding the trusses to the pipeline. Further, the gross weight of the system is increased significantly and may result, in extreme cases, in one or more of the mobile tower assemblies becoming mired in mud or wet sandy soil as the system trasverses an already partially irrigated field.